Continuous fiber reinforced composites (CFRPs) are extensively used in both primary and secondary aircraft components for a variety of applications where light weight, higher strength, and corrosion resistance are primary concerns. Composites are typically composed of fine carbon fibers that are oriented at certain directions and surrounded in a supportive polymer matrix. Since the plies of the composite material are arranged at a variety of angles, and depending upon the direction of major loading, the resultant structure is typically a stacked laminated structure which is highly anisotropic and heterogeneous. A significant portion of the composite structure is fabricated as near net-shape, but is drilled in order to facilitate joining of components using mechanical fasteners. Drilling fastener holes in composite does not compare to the uniformity of aluminum or steel, since individual carbon fibers fracture at irregular angles and form microscopic voids between the fastener and the hole. As the cutting tool wears, there is an increase of surface chipping and an increase in the amount of uncut fibers or resin and delamination. The composite microstructure containing such defects is referred to as “machining-induced micro texture.”
In addition to their machining challenges, composite structures in aircrafts are more susceptible to lightning damage compared to metallic structures. Metallic materials, such as aluminum, are very conductive and are able to dissipate the high currents resulting from a lightning strike. Carbon fibers are 100 times more resistive than aluminum to the flow of current. Similarly epoxy, which is often used as a matrix in conjunction with carbon fibers, is 1 million times more resistive than aluminum. The composite structural sections of an aircraft often behave like anisotropic electrical conductors. Consequently, lightning protection of a composite structure is more complex due to the intrinsic high resistance of carbon fibers and epoxy, the multi-layer construction, and the anisotropic nature of the structure. Some estimates indicate that, on average, each commercial aircraft in service is struck by lightning at least once per year. Aircraft flying in and around thunderstorms are often subjected to direct lightning strikes as well as to nearby lightning strikes, which may produce corona and streamer formations on the aircraft. In such cases, the lightning discharge typically originates at the aircraft and extends outward from the aircraft. While the discharge is occurring, the point of attachment moves from the nose of the aircraft and into the various panels that compromise the skin of the aircraft. The discharge usually leaves the aircraft structure through the empennage.
The protection of aircraft fuel systems against fuel vapor ignition due to lightning is even more critical. Since commercial aircraft contain relatively large amounts of fuel and also include very sensitive electronic equipment, they are required to comply with a specific set of requirements related to lightning strike protection in order to be certified for operation. Fasteners are often the primary pathways for the conduction of the lightning currents from skin of the aircraft to supporting structures such as spars or ribs, and poor electrical contact between the fastener body and the parts of the structure can lead to detrimental fastener-composite effects such as arcing, sparking, internal plasma formation, high surface temperatures, thermionic electron emission, and large vapor pressures.
To avoid these detrimental lightning initiated effects at the fastener-composite structure interface, some aircrafts use fasteners which are in intimate contact with the fastener and CFRP hole. Intimate contact between a bare metallic fastener and the hole in the composite structure has been known to improve electrical current dissipation. One approach to achieve fastener-to-composite hole intimacy is to use a sleeved fastener. This approach involves first inserting a close fitting sleeve into the hole. An interference-fit pin is then pulled into the sleeve, which expands the sleeve to bring it in intimate contact with the CFRP hole surfaces in the composite structure. Although sleeved fasteners substantially reduce the gap between the fastener and composite structure, it cannot eliminate the small gaps created due to presence of drilling induced micro texture on the inner hole surfaces. Machining induced texture also entraps excess fuel tank sealant, an insulating dielectric material, inhibiting intimate contact between the sleeve and hole. This situation becomes worse as the cutting tool wears resulting in more surface irregularities and larger machining induced surface defects. In addition, these larger sized holes need to be drilled to accommodate additional sleeve thickness, thus resulting in heavier structures.
In order to mitigate these types of lightning induced conditions, the high amplitude transient currents must be distributed throughout the carbon fiber structure and copper mesh embedded on the surface, with the majority of current flow occurring perpendicular to the fastener hole due to the anisotropy of the CFRP resistivity. If the fastener is not in intimate contact with the inside of the hole, the Joule heating energy contained within the frequency dependent skin depth regions will result in melting of metal surface layers and adjoining sealant layer, thus producing high vapor pressure regions. A typical lightning discharge can deliver 10-100 Coulombs of charge, which results in large voltage differentials across dielectric layers and gap regions. These high electric fields result in voltage breakdown phenomenon which is accelerated by increased vapor pressure (higher particle density) and results in arcing and spark formation. These effects are the catalyst for the formation of internal plasma (ionized gas) which reaches high temperatures and internal pressures within the volume between the fastener and hole. The intrinsic high conductivity of metallic fasteners and the large number of fasteners used in aircraft construction combine to create a condition of high probability of lightning attachment to fasteners and the formation of these effects.
In the development of new aircraft and changes in regulation requirements regarding lightning protection, it has become imperative that fastener designs are needed for aircraft structural areas which are unable to accommodate a sleeved fastener system. In many situations, the size of holes and proximity of fasteners is restricted due to mechanical limitations and thus alternative fastener designs are essential for lightning strike protection. The distribution of lightning current is highly dependent on establishing a good electrical contact between the fastener and CFRP. In the majority of composite systems, an interference-fit between the fastener and hole during installation results in additional breaking of carbon fibers and large shear forces that results in delamination and failure mechanisms (cracking) within composite layers.